1. Field of the Invention
The present invention relates generally to a variable damping force shock absorber for an automotive suspension. More specifically, the invention relates to a shock absorber which has variable damping characteristics relative to stroke of relative displacement between a vehicular body and a suspension member rotatably supporting a road wheel for enhanced damping performance.
2. Description of the Background Art
Japanese Patent First Publication 63-203939 discloses a variable damping force shock absorber which is variable of damping characteristics depending upon piston stroke. In the disclosed construction, a spring disc is provided for openably closing a fluid flow path formed through the piston for providing restriction for fluid flow therethrough and thus generating damping force. A coil spring is associated with the spring disc. In the shown construction, the coil spring is active on the lower spring disc for exerting spring force onto the spring disc during piston bounding stroke. As can be appreciated, the spring force to be exerted to the spring disc by the coil spring increases according to increasing of the piston stroke in bounding direction. On the other hand, the spring force as integrated force of the spring disc and the coil spring is active to depress the spring disc in a direction for closing the fluid flow path for permitting working fluid to flow from an upper fluid chamber to a lower fluid chamber. Therefore, at the initial stage of piston rebounding stroke subsequent to the piston bounding stroke, fluid force overcoming the integrated spring force is required. By this, at the initial stage of the rebounding piston stroke, increased magnitude of damping force can be obtained. According to expansion of the coil spring due to piston rebounding motion, the spring force to be exerted on the spring disc is gradually decreased. Such variation characteristics of the damping force assures vehicular driving stability.
In such conventional shock absorber, since higher response to the piston stroke is required, the coil spring is constantly in contact with the spring disc in such a manner that the spring force exerted to the latter can be approximately zero at the piston neutral position. This means that the increased spring force is generated in response even to substantially small stroke of piston vibration. As a result, the integrated spring force of the coil spring and the spring disc is active for restricting deformation of the spring disc. Combination of the coil spring and spring disc may cause substantial increase of the spring coefficient which leads degradation of riding comfort.